For the contact lens wearer who requires presbyopic (or near vision) correction, in addition to distance correction, a variety of options have been available. These individuals may be fitted with single vision contact lenses corrected for distance, and wear reading glasses for near correction. Another alternative is to provide a contact lens for one eye that is corrected for distance vision and to provide a contact lens for the other eye that is corrected for near vision (this practice is referred to as monovision because only one eye is corrected for near vision), or the fitting of bifocal or multifocal contact lenses.
During the 1950's, a variety of contact lenses were designed for the correction of presbyopia. These contact lenses, although very innovative in design, met with only limited success because the only readily available material was Poly Methyl Methacrylate (Plexiglass), also known as P which does not transmit oxygen. As bifocal and multifocal designs of the period were quite thick and heavy compared to conventional distance correction contact lenses, these presbyopic contact lenses were uncomfortable to wear for substantial periods of time. Additionally, the fitting of these bifocal and multifocal contact lenses required considerable time and skill on the part of the contact lens fitter.
During the 1970's, both soft contact lenses and rigid gas permeable (RGP) contact lenses were introduced. With the availability of these new materials, renewed enthusiasm brought about several new designs for contact lenses for the correction of presbyopia.
RGP materials provide oxygen transmission through the lens material itself, and afforded new hope for the earlier designs developed in PMMA material. However, lens thickness and resultant patient discomfort continued to be a problem.
One of the early benefits recognized with soft contact lenses was the comfort and ease of fitting and, for this reason, by 1995 approximately 85% of new contact lens wearers are being fitted with soft contact lenses. As soft contact lenses command such a large share of the contact lens market, it is natural that considerable effort would be made to develop bifocal and multifocal contact lens designs in soft contact lens material.
There are two types of contact lens designs for the correction of presbyopia - - - Alternating (or Translating) and Simultaneous.
(1) In the alternating (or translating) vision technique, the lenses are very similar in design to bifocal eyeglass lenses in that the wearer sees through the distance segment in the upper portion of the lens when looking straight ahead and sees through a lower near vision segment when the eye (moves) to look down. Alternating vision lenses have proven to be successful in RGP designs, but have met with little success when designed in soft contact lenses.
Perhaps the reason that alternating vision soft contact lens designs were not as successful at the same design concept in RGP materials was because lens translation is necessary for this design to be successful. The translation from distance to near is achieved through the mechanical action of the lens resting on the lower eyelid and, when the eye looks down, the lens remains stable on the lower eyelid causing the pupil of the eye to translate from the distant vision portion of the lens to the near vision portion of the lens. Soft lens material by its nature caused this modality to fail as there was insufficient rigidity in the soft lens to remain properly positioned on the lower eyelid and often the lens would slip underneath the lower eyelid during translation.
(2) Simultaneous vision bifocal or multifocal contact lenses are either concentric or aspheric in design with focal power changing through different areas of the lens. Lenses are fitted so that distance, intermediate and near zones focus images simultaneously on the retina of the eye and the brain then separates out the image desired.
Theoretically, with adaptation, the ability to change focus naturally from near to far with no blurring in between can be achieved with simultaneous vision lenses in both RGP and soft contact lenses.
As alternating presbyopic designs proved to be unsuccessful in soft contact lens designs, most of the development work with soft contact lenses was done in the area of simultaneous presbyopic correction with concentric designs or aspheric designs.
During the 1980's, several designs of concentric and aspheric soft contact lenses were introduced. Soft aspheric multifocal contact lenses typically provided relatively weak reading addition power and therefore worked best in early presbyopia.
Reading addition powers are referred to by eye care professionals as "add" power, and represent the difference between the distance correction and near correction prescribed by an eye care professional for eyeglasses or contact lenses. Accordingly, a prescription of "-3 with a +2 add" (which would be typical for moderate presbyopia) would mean that distance vision requires -3 diopters of correction, and near vision requires an additional 2 diopters of plus correction, resulting in -1 diopters of near vision correction. In conventional monovision, the dominant eye would be fitted with a -3 distance correction lens, and the other eye would be fitted with a -1 near correction lens.
This type of solution is often satisfactory in early presbyopia because the user still has some remaining visual accommodation and the needed add power is usually between +0.75 and +1.25, which is usually low enough for the brain to comfortably select the desired image in most people. However, conventional monovision becomes less satisfactory as presbyopia becomes more advanced because the needed add power increases and visual accommodation has deteriorated further, so that the visual imbalance exceeds the brain's ability to select the desired image from the appropriate eye.
Typically, early presbyopes, would be between the age of 40 and 45, and would require add power of between +1.00 and +1.50 diopters. Moderate presbyopes would usually be between 45 and 55 years and would require add power of between +1.50 and +2.00 diopters. Mature presbyopes would usually be older than age 55 and require an add power of between +2.00 and +3.00 diopter.
The add corrective power of current aspheric multifocal contact lens designs is usually limited to only +0.75 to +1.25 diopters because the brain must be able to separate out the desired image (and also suppress the undesired images) from the multiple images (near, intermediate or distant) being simultaneously focused by the multifocal contact lens design. In order to achieve this suppression, the images cannot be too different from each other. However, if aspheric corrections are increased in attempts to achieve higher add powers, the images become too different for the brain to suppress the undesired images, resulting in blurred vision. Even at add powers of +0.75 to +1.25 diopters, many patients suffer some blurring or ghosting with multifocal contact lens designs because their brains are not able to completely separate the desired image while simultaneously completely suppressing the undesired images.
Some contact lens fitters may attempt to use aspheric designs to achieve near distance correction of up to +2.00 diopters (or more) by undercorrecting the distance vision of the non-dominant eye by between 0.25 and 1.00 diopters, thereby theoretically providing up to +2.00 diopters (or more) of near vision correction, instead of the +0.75 to +1.25 diopter correction that would be provided if that eye had been fully corrected for distance vision with an aspheric multifocal contact lens. The dominant eye would be corrected to maximum distance acuity in such a situation. However, this creates even more blurring and ghosting. This technique is called modified monovision.
Aspheric optics have been incorporated on both the front and back surfaces of soft contact lenses. However, it is believed that front surface aspherical multifocal soft contact lenses provide better presbyopic correction. Still, only limited success is achieved because providing add power of +0.75 to +1.25 (or more) usually results in reduced distance acuity. For this reason, many contact lens fitters find it necessary, when using aspheric soft multifocal contact lenses, to undercorrect the distance power in one eye to improve near vision, while correcting the other eye fully for distance vision, as discussed above. When attempting to fit moderate to mature presbyopes, this modified monovision almost always results in a visual compromise similar to that of conventional monovision.
Concentric multifocal lens designs have an advantage over aspheric designs in the fitting and correcting of more mature presbyopes requiring add power of more than +1.25 diopters, primarily due to the availability of higher add power correction and central power zones of different diameters. Concentric soft multifocal contact lenses have been made with the central distant correction zones and central near correction zones. In the latter designs, the central power zones would be corrected by the amount prescribed to correct near vision. It is believed that central near add zones have been more successful at correcting presbyopia than central distance zones, when incorporated in concentric multifocal soft lens designs. Although concentric center add multifocal designs have the ability to correct higher add power requirements, most individuals fitted with this type of lens experience moderate to significant amounts of visual discomfort due to ghosting of images or a 3-D effect, at near distances. These effects diminish with adaptation, but still cause a high portion of wearers to discontinue the use of this type of presbyopic contact lens.
The reality of the existing art of presbyopic correction with simultaneous vision contact lenses is that no currently available lens system, be it aspheric or concentric, provides monocular multifocal correction for moderate to mature presbyopia. In most cases, some form of modified monovision is required in an attempt to satisfy the visual requirement for near and far vision. To this end almost all currently available presbyopic contact lens manufacturers indicate in their fitting manuals the requirement of compensating one eye more for near and the other eye more for distance correction. This is the norm rather than the exception. Additionally, no currently available multifocal contact lens has the ability to restore the phenomena of natural accommodation and successful results are difficult to achieve and require considerable time and experience on the part of the fitter.
It is therefore an object of this invention to provide true multifocal correction for moderate and mature presbyopes requiring up to +3.00 diopters of add power without the need to compensate one eye for near and the other eye for distance.
It is a further object of this invention to provide rapid patient adaptation with minimal initial visual discomfort.
It is a still further object of this invention to provide a presbyopic optical system that restores the phenomenon of natural accommodation.
It is a still further object of this invention to provide a system of fitting and methodology that allows a contact lens fitter with little or no multifocal contact lens fitting experience to achieve a very high degree of success and patient satisfaction.